fit. you should be comfortable and the bike should fit, it should feel good. your kid said he liked the feel of the one bike over the other, so that is the bike for him.

after that, make sure it's a good bike, i.e. it is made by a reputable bike company (Next is not a good bike, Specialized is).

this bikes you listed are all good bikes, so the only thing you need to focus on is fit.